Unless there is a reason that is not evident to most at the present time.
That be as it may (and I guess we will see)....
I strongly feel going purely by US perspective solely (which as you know I am aligned with in various ways...some more than others) there was severe botching by Biden admin from executive side. Given:
A) Initial rationale for the intervention w.r.t sobering appraisal of CJCS Milley et al in congressional hearing lately as to the nature of the void and the high probability of AQ and other extremist groups filling it within the next 2 years.
B) The sheer scale of betrayal heaved onto those left behind (those gathering in Kabul airport are a tiny fraction) that trusted in the US and worked for them in all manner of capacity (be it the primary or secondary goals).
Given the US has been there 20 years, surely it would have gotten more return on what it invested in those 20 years by gradual exit over 1 - 2 years more?...and keep the "not evident reasons" (i.e long term strategy) fairly the same as well.
Consider what the lines were that were being held with 3k grunts and 2k contractors already (even after the 13k grunt withdrawal done by Trump)....and the last US casualty being in Feb 2020 (clearly showing the brunt had been transferred to ANA a long while ago and US was more a protection detail for its very important contractors providing crucial onshore support for critical ANA operations and deterrence).
i.e Wouldn't a withdrawal with proper accommodation given to stuff like contractors that are so key to ANA and which they grew reliant upon....have been far better from any angle you get at this?
It is this disregard (by the exec against all intel and advisors on the matter from what I can tell) that has most surprised me....and now Obama and Gates words about Biden ring all the more clear. Combine this with the totally tone deaf flat out lies the US president has been saying (knowing fully well what intel he overruled) in the last month and now when the situation unfurled this way inevitably (by own intelligence shown).
Paraphrasing from a buddy of mine (retd. USMC major)...who saw things there up close during his tours:
Were 2000 (extremely crucial) contractors and 3500 support grunts worth stripping away like has been done?.
Given what they were holding and supporting w.r.t key US interests?...and lives and resources used to get there over 20 years?
At least a somewhat planned staggered withdrawal (with checks and balances on how the ANA function without x y and z and how the taliban respond to that) could have mitigated a lot?
The last US casualty was in february 2020 for reference as to who was taking the far larger brunt (it was the ANA ever since 2014 especially).
The ANA and security forces have lost 50,000 soldiers in this conflict (fighting the Taliban), that is no small number...and highlights what exactly was taken from them in short order of a few months....critical things that they had grew accustomed to and grown reliant on (like Close Air support MRO contractors).